The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply, it includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment.
Alternative sources for developing combustible liquid fuel are desirable. An abundant source is natural gas. The conversion of natural gas to combustible liquid fuel typically involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they typically do not contain appreciable amounts of nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have less health and environmental impact than conventional petroleum-based fuels. Fischer-Tropsch synthesis is a preferred means for converting syngas to higher molecular weight hydrocarbon products.
Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C20+wax, which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Although some catalysts have been developed with selectivity for longer chain hydrocarbons, the hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable.
It would be advantageous to provide a method for hydroprocessing Fischer-Tropsch wax which minimizes the hydrocracking of hydrocarbons in the distillate fuel range. The present invention provides such methods.
The present invention is directed to a method for hydroprocessing Fischer-Tropsch products. The invention in particular relates to an integrated method for producing liquid fuels from a hydrocarbon stream provided by Fischer-Tropsch synthesis. The method involves separating the Fischer-Tropsch products into a light fraction with normal boiling points below 700xc2x0 F. and including predominantly C5-20 components and a heavy fraction with normal boiling points above 650xc2x0 F. and including predominantly C20+components. The heavy fraction is subjected to hydrocracking conditions, preferably through multiple catalyst beds, to reduce the chain length. The products of the hydrocracking reaction following the last hydrocracking catalyst bed, optionally after a hydroisomerization step, are combined with all or a portion of the light fraction. The combined fractions are hydrotreated, and, optionally, hydroisomerized. Hydrotreatment hydrogenates double bonds, reduces oxygenates to paraffins and desulfurizes and denitrifies the fractions. Hydroisomerization converts at least a portion of the linear paraffins into isoparaffins.
When the products of the hydrocracking reaction have passed through the last bed of hydrocracking catalyst, they are at a relatively elevated temperature. When these products are combined with the light fraction, the light fraction acts as a quench fluid, cooling the fraction, preferably to a desired temperature for performing the hydrotreatment step.
In one embodiment, the light fraction is introduced into a reactor at a level below the last hydrocracking catalyst bed and above or within a hydrotreatment bed. In this embodiment, the temperature and/or pressure of the hydrotreatment bed can be, and preferably are essentially the same as that in the hydrocracking bed(s). In another embodiment, the products from the hydrocracking reactor are pumped to a separate hydrotreatment reactor, where they are combined with the light fraction. In this embodiment, the temperature and or pressure of the hydrotreatment reactor can be, and preferably are different than that in the hydrocracking reactor.
The products from this xe2x80x9csplit-feedxe2x80x9d hydroprocessing reaction can be separated into at least a hydrogen-rich gas stream, a distillate product predominantly in the C5-20 range, and a bottoms stream. The bottoms stream can optionally be resubjected to the hydrocracking conditions to provide an additional light fraction, or used, for example, to prepare a lube base stock.
In one embodiment, the heavy fraction and/or the light fraction include hydrocarbons in the same range derived from other sources, for example, petroleum refining.